This invention relates to a magnetic bearing control arrangement including a closed-loop rotor position control system and a separate open loop vibration correction system. The use of multiple systems to perform separate tasks maximizes the dynamic response of the control arrangement.
A number of papers have been published which discuss possible algorithms for implementing open-loop control methods for correcting rotor system mass imbalance in rotating devices having magnetic bearings. As described in Knopse, C. R., Hope, R. W., Fedigan, S. J. and Williams, R. D., "Adaptive On-Line Rotor Balancing Using Digital Control," Proceedings of MAG '93 Magnetic Bearings, Magnetic Drives and Dry Gas Seals Conference & Exhibition, Alexandria, Va., Jul. 29-30, 1993, Technomic Publishing Company, Inc. and Ku, C. -P. R. and Chen, H. M., "Optimum Shaft Balancing at a Rotor Bending Critical Speed with Active Magnetic Bearings", Proceedings of MAG '93 Magnetic Bearings, Magnetic Drives and Dry Gas Seals Conference & Exhibition, Alexandria, Va., Jul. 29-30, 1993, Technomic Publishing Company, these algorithms use signals acquired from vibration transducers on the support structure for the magnetic bearings, as well as the rotor position sensors used by the magnetic bearing controller for closed-loop rotor position control, to derive sinusoidal perturbation signals that are applied to both the vertical and horizontal axes of each radial bearing in the system. These sinusoidal perturbation signals generally have equal amplitude and frequency, but have a 90.degree. phase relationship to each other and act similarly to a balance weight inserted into an imaginary balance plane at the same axial location as the radial bearing(s). Fedigan, S. J., and Williams, R. D., "An Operating System for a Magnetic Bearing Digital Controller," Proceedings of MAG '93 Magnetic Bearings, Magnetic Drives and Dry Gas Seals Conference & Exhibition, Alexandria, Va., Jul. 29-30, 1993, Copyright 1993 by Technomic Publishing Company, Inc. discusses real time operating systems that might allow a single digital signal processing (DSP) unit to perform the continuous task of closed-loop rotor position control for a multi-axis bearing system while still leaving time between program loops to recalculate the necessary amplitude and phase angle for the open-loop sinusoidal perturbation signals.
However, a system using two separate DSP units; one for the high priority task of closed-loop rotor position control, the second for the lower priority but computationally intensive task of recalculating amplitude and phase for sinusoidal perturbation signals, would avoid any priority conflicts and has performance advantages over a single DSP unit. This type of system, coupled with a homopolar bearing as disclosed in U.S. Pat. No. 5,111,102 to Meeks, or other bearings having redundant control coils, could also be used to implement a completely redundant magnetic bearing system/controller with both digital closed-loop rotor position control and open-loop mass imbalance correction and harmonic vibration control.